It also fired an artillery barrage into a maritime "buffer zone", in what experts said was part of an "aggressive and threatening" response by Pyongyang to the large-scale joint air drills the United States and South Korea are currently conducting.South Korea closed some air routes over the East Sea, also known as the Sea of Japan, advising local airlines to detour to "ensure passenger safety in the routes to the United States and Japan".In November the same year, the North shelled a South Korean border island, killing two marines -- both of them young conscripts.The test follows a recent blitz of launches, including what the North said were tactical nuke drills, that Washington and Seoul have repeatedly warned could culminate in another nuclear test -- which would be Pyongyang's seventh."As far as I can remember, North Korea has never made such a provocation when South Korea and the US were holding their joint drills," Park Won-gon, a professor at Ewha University, told AFP."